Truck dies when hot and then runs later

[mike_m]

My M37 seems to have the classic symptoms of a bad coil, but I'm interested to see if someone on the forum has another idea. I drove the M around for about a half hour today. It just suddenly died like the ignition switch turned off. I didn't have any other symptoms like missing, sputtering or anything else. I did verify there was fuel in it. After about 10 minutes it started right up. The coil is somewhat new and I have verified that the lines and fitting are correct. I was wondering if a faulty circuit breaker could cause this or perhaps a condenser. Unfortunately when it died, I didn't have my timing light with me, so not sure if it had spark when it happen, but base on my experience, I think it didn't have spark. Just looking to see if there could be another possibility besides the coil. I don't drive the truck very much at all since I've been very slowly getting back together. Appreciate any ideas or advice.

I do have a spare coil and condenser and can try those out the next time it dies!

Thanks

Mike M

[RMS]

haven't changed a coil yet, had the truck for 16years..... who knows how old it is... Ive put on 50,000+

one fall years ago I did have the truck die a few times. then after sitting start back up. a few times the carb iced up then the latent heat would melt the ice and the truck would then start. but twice she died with no known cause. no frost on the carb, no tools in hand........

I like your thinking on the timing light...... I thought my timing light died the other day, hooked it to #1 no flash checked for spark at the end of the lead, all was good. swapped wire still no flash, pulled plug...... oil fouled. dropped in a new spark plug and the strobe was flashing.

hmm not long after I replaced the old genny with a modern 24v 100amp unit. ..... dirty power messing with the coil and condenser ?

[mike_m]

Thanks for the suggestion. I should add that I did walk back home after it died, got my timing light and it did have spark and started. I didn't check the plugs, but doubt that's the issue. If I had to guess, I think its crappy foreign coil, but I really won't know until it dies again and I try another one. Time will tell.

[RMS]

I was thinking it takes more jam to make a spark under compression and a timing light could confirm an intermittent weak spark.

[m-11]

it could be the condensor or the coil. If you have good spares I would swap one out at a time and see if anything changes.

[mike_m]

All-
Thanks for your replies/suggestions. I'll follow up with this post when I figure out the issue.

[mike_m]

Follow up:

Drove the M around again, though this time I had my timing light. Sure enough after about a half hours worth of driving around the block the vehicle died. This time it did sputter a tiny bit and then died. Got out the timing light and hook it up. To my surprised it DID have spark and in fact started but didn't run. Verified gas in the carb as well.
I did try several times and it would start up but not run. Waited for about 15mins and it started right up and ran fine.
I'm a little stumped what's a going on. I do have spare parts and will put in a new condenser and go from there. Any other ideas? I'm still convinced its heat related and electrical.
Any ideas appreciated.
Thanks

Mike M

[Cal_Gary]

If you still have spark, I'd check to see that the gas tank is properly vented. Vapor lock causes similar symptoms, creating vacuum because the tank can't breathe as fuel is comsumed....
Gary

[Kaegi]

is fuel line too close to exhaust manifold? fuel pump dying? coil might still show some spark and be failing. the spark might be so weak from a bad condenser, bad coil bad or improperly set points that the spark is getting blow out under compression. I had a strange one about 8 years ago in my 66 D300 with poly 318. It had a rebuilt dist the PO had installed and it would nor run after about 20 minutes or not start after driving for a while then say shut off to get gas no restart. the screw holding the points down was waaaay too tight and when the engine got hot it would deform the plate and post and points were sticking open. loosened screw and tightened to normal and no probs after that. not suggesting that's your case but just shows anything can happen sometimes.

[MarkPick]

I had a similar problem, folks on this board suggested a compression test of each cylinder. Found out I had one that was worse than the others but none were what you could call good.

[m37jarhead]

I know you said you saw fuel in the carb. But I would still follow the fuel starvation thread. Vapor lock, poor pump pressure, clogged fuel filter, etc., etc. If you have spark try using a burst of starter fluid.
The engine would likely die only from lack of spark or lack of fuel. I agree that your problem is related to engine heat.
Vapor lock or hot ignition parts. You COULD temporarily reroute the fuel line from the pump to the carb to get it away from the hot exhaust manifold. If it works, it would sure point a guilty finger at a fuel line too close to the manifold.

Engines don't just die from fouled plugs or poor compression.... they die a slow death from that.
Jerry

[mike_m]

Thanks for all the suggestions. I haven't had time to look at it lately but perhaps this weekend.
Stay Tuned

[mike_m]

Hi Gang,

Just wanted to follow up with what I found. After taking my distributor cap off, I noticed that the tab that protrudes out to the coil was cracked all the way around where the spring is. After replacing my distributor cap with a new one, the issue of my truck suddenly dying after running for about 15mins went away. Hopefully this might help out someone else in the future who is experiencing similar problems.

Thanks

Mike M

[Kaegi]

good find. I have had that happened on some old cars before. the crack gets wider when hot then the spark shorts out or starts jumping around causing all kinds of issues. sometimes your cant even seethe crack with the naked eye.

[mike_m]

I was fortunate that I did see the crack, otherwise I thought for sure I my new coil was bad. It's been challenging/rewarding experience. Took a while to figure it out.